Seems most of the info for solving this is in the above posts or there abouts.
Paul any hair spray would have evaporated after the spillage, any residue left would have been cleaned a way by the thorough clean and prep you have carried out to the service.
It is a grease problem you have and there could be a couple of things adding to the problem.
1. Is the customer on any medication. I had a real bad job a few years ago with similar problems, the customer had a heart condition and was on heavy medication, he also sweated alot he used to rub his hands on the end of the chair arm and this over the years removed the top coat ans some colour. I had similar problems to you doing this job, but you can get around it.
If not then
2. It is a grease problem and the customer may have cleaned it in the past with every thing under the sun and that is causing the contamination problem.
Basically 2 things will happen when you do a job like this , one is that the paint and top coat will dry ( some times a little taccy and some times not) then hours or days later it will break down again with the contamination of grease/ contaminates and go soft/sticky and some times even smelly.
The second thing that can happen is the reverse of the above and it will not cure out correctly and harden but still break down give a micro orange peel effect to the finish and make it feel rough with a slight sticky like touch to it. This can also happen if the leather surface itself is starting to breakdown.
It is a tricky one to solve but quite straight forward.
It would take me too long to post/type how to fix this problem, but feel free to contact me any evening and I will explain how.
Paul